This invention refers to an electrolytic cell for the generation of hypo-halogenites in a continuous process.
The electrolysis of halogenites in a liquid medium producing acid hypo-halogenites or their corresponding salts has been employed for disinfection treatments of water in general, especially for water in swimming pools, cooling towers and reservoirs, including drinking water and waste water.
There are several types of electrolytic cells for this end, installed as part of a hydraulic system which the liquid to be treated flows through (on-line electrolytic chlorinators). The water being treated may contain a natural residue of a halogen salt, usually sodium chloride; if necessary, an appropriate amount may be added. Within the cell are appropriate electrodes that communicate externally to a continuos current electrical supply. The electrolysis of the halogen salt occurs on the surface of the electrodes, and the corresponding halogen (chlorine, if the halogen salt is sodium chloride; bromine, if it is sodium bromide, and so on), hydrogen and, to a lower degree, oxygen, are formed.
As there is no separation between the anode and the cathode, the halogen combines with the medium to form the corresponding hypo-halogenites. The products of the electrolysis are taken outside the cell by the flow of water, and the hypo-halogenites is dissolved. The hydrogen and the oxygen, not very soluble in the medium, are expelled to the atmosphere by the exposed surface of the liquid. This type of equipment is generally known as hypochlorite generator, electrolytic chlorinator, salt water chlorinator or electrolyser.
Due to the low salt content dissolved we will occasionally refer to this solution quite simply as water or electrolyte, it being understood that it is an electrolytic solution, that is, water with a certain amount of salt or salts of one or more halogens. The first electrolytic cells of the type were built in segments of commercial plastic pipes with concentric tubular electrodes or electrodes in flat parallel grid or solid plates, attached to their inside, and electrical lead linking them to an external supply through sealed openings on the wall of the cell. The cells were installed in commercially available plastic pipes that are part of the system of the water to be treated.
A problem common to this type of equipment was the possibility of energizing the cells with the pump switched off, that is, without any water flowing through the cell. In this case, the operation of electrolysis can cause an accumulation of hydrogen and oxygen gases--a highly inflammable and explosive mixture--inside the hydraulic system, such as filters, pumps and other containers close to the electrolysis, with a high risk to people and facilities.
A few devices were added to the technique to prevent this possibility. We will mention some of these devices, still used with certain well-known equipment. One of these involves the installation of a flow meter in the pipes, interrupting the electrical supply of the electrolytic cell if the flow of water within the cell is interrupted. However, common flow meters usually have switching problems after a certain time, as their electrical contacts--very close to the flow of the electrolytic solution--are easily oxidized by steam and halogens.
Another solution for the problem was the design and adoption of a shunt shaped like an inverted "U" placed vertically in the water pipes. The cell can be placed in one of the segments of the inverted "U"; its set of electrodes must be higher than the entry and outlet level of the water. If there is no flow of water to conduct the hydrogen and oxygen thus formed outside the cell the electrolysis will only produce the required amount of these gases required to fill in the shunt and thus eject the electrolytic solution that remained within the cell. If there are no electrolytes close to the electrodes the electrolytic process is interrupted.
In the absence of a term that better defines it and for greater clarity we will henceforth call this device the "capacity of self-extinction of the electrolysis".
The shunt is made by the person installing the electrolytic cell; this person is not effectively controlled by the equipment manufacturer. The shunt may later suffer alterations because of mending or changes in the system of pipes. Therefore an improvement of the device, described by U.S. Pat. No. 4,861,451, included this device in the actual electrolytic cell, that is, giving the cell the shape of a pipe like an inverted "U" where the electrodes are on the horizontal part. The entry and outlet of the electrolyte occurs in the two vertical pipes that are also used to connect it to the hydraulic system. This configuration leads to the production of a necessarily large facility for a relatively small electrode area.
Another variation of the "capacity of self-extinction of the electrolysis" was added to the electrolytic cell model described in the request for Australian patent nr. AU-A-1110/88. It was described as a chamber formed by an upper and by a lower part. The entry and the outlet of the electrolytic solution occurs on the lower part, placed horizontally in diametrically opposing points, or vertically on the lower part at opposing ends, as in the model previously described, separated only by a dividing wall. The upper part holds a compartment for the set of electrodes (ascending flow electrode chamber), also separated by a division but only until the height of the electrodes. Above this height the chambers communicate. These electrodes, which can be flat plastic plates both entire or grid-like, can be placed both vertically and longitudinally in relation to the flow, that is, they may be perpendicular and adjacent to the division, or vertical and across the entry of the solution. In this case the last electrode next to the effluent chamber (necessarily a smooth flat plate) will substitute the upper division. When we juxtapose the upper and the lower parts we have a single recipient, with vertical divisions that combine to form a sort of dam. When the electrolyte flows through the cell it must cover the set of electrodes and flow over the upper division before it leaves through the outlet compartment (effluent chamber), which only serves to drain the solution. Electrolysis does not occur in this chamber.
This type of solution involves an increase in the physical size of the cell, that ends up by incorporating, in addition to the electrodes (electrode chamber) an additional empty vertical compartment (effluent chamber) dedicated to the flow of liquid.
The two above-mentioned configurations of the internal or external additions of the "capacity of self-extinction of the electrolysis" in the cell imply, on one hand, a smaller relation of the area of electrodes per volume of electrolytic cell or volume occupied by the installation and, on the other hand, an increase of potential danger should the cell become full of hydrogen and oxygen, since it can contain more gas.
We should emphasize that Australian regulations (Queensland Gas Examiner--1881), accepted and adopted by SOASA--Swimming Pool and Spa Association of Australia Limited, according to the Safety Guide for Electrolytic Chlorinators (Electrolytic Saline Chlorinator Safety Guidelines, Esc-2, 1989), has determined that the amount of accumulated gas in a hydraulic facility with this type of cell--that is, throughout the inverted U-shaped shunt--cannot be more than two liters. This requirement, therefore, limits the volume of the aquatic systems to be treated--swimming pools or cooling towers. They can be treated with equipment built according to the technique described above as long as it complies with such regulations.
Another proposal developed not to substitute but to complement the safety of the electrolysers in order to avoid that the installation fill up with gases has an electronic gas detection system that measures the conductivity between an auxiliary electrode located internally at the top of the cell (gas sensor) and one of the main electrodes (anodes or cathodes). Should there be an accumulation of gases within the cell, the conductibility--guaranteed by the liquid within the cell--is considerably reduced due to the low electrical conductivity that characterizes non-metallic gases, and the source will cut the supply of electrical energy to the electrodes. However, as the auxiliary electrode sensor is fed by a continuos current, it may undergo a polarization that can inhibit its sensing capacity.
Electrolytic cells fed with continuos current need periodical cleaning to remove eventual deposits of calcium salts and other minerals present in the water from their cathodes, as the presence of such salts considerably reduces the efficiency of the electrolytic cells considerably. The frequency of this cleaning operation depends on several factors, the most significant one being water hardness, which usually occurs because of the immersion of the cathodes in an acid solution. In several types of electrolysers this operation is facilitated by the incorporation of a device to remove and replace the parts to be treated into the design of the cell.
Some types of equipment allow the removal of the set of cathodes, while others with anodes and cathodes as a single block or beam only allow the removal of the entire set of electrodes. When the entire set is removed for cleaning the anodes (with a valuable and fragile coating of noble metals) are unprotected. Removing and replacing only the cathodes alleviates the problem, but subjects the valuable fixed anodes to scratches and damages from the cathodes when the operation is executed, especially when removing cathodes with thick mineral crusts that may also be attached to the anodes from the cells.
The electrical safety of these units is vitally important. In some models the cathodes are grounded, that is, the grounding is executed by the same electrical cable that takes the current to the cathodes. As this equipment works with relatively high currents, any fault in connector contact eliminates the grounding from the unit. This can be fatal for any living being that comes into contact with the electrolytic solution, even if the difference in potential is low.